By the time concrete is produced and placed on a project, the engineering and design phases are usually complete. The codes and standards used by the designer to develop the contract documents for the project may therefore seem to be of little importance to constructors. But improvements to the concrete building code that address the use of new materials and placement techniques may promote cutting-edge construction practices—ones that will drive procedural changes for concrete producers and contractors.
The newest version of the Building Code Requirements for Structural Concrete, ACI 318-19, was released by the American Concrete Institute (ACI) in July 2019. Changes that could impact concrete production and construction include new provisions for the use of alternative cements and alternative aggregates, shotcrete, high-strength reinforcing steel, and post-installed screw anchors. This new edition of the ACI 318 Code also includes new details for seismic resistance and specifies training and certification for inspectors.
The Code does not, however, include design criteria and expected performance for new alternative materials. Not enough industry testing has been done so far, so responsibility is placed on the material suppliers (see page 38). The solution, at least for now, is to rely on the design team, materials suppliers, and concrete producers to perform testing and provide data on the expected performance of these products.
The concrete industry has been under pressure to allow alternatives to portland cement, motivated by one of three main drivers (according to ACI ITG-10R-18, Practitioner’s Guide for Alternative Cements):
1. Reduced cost—both initial cost and life-cycle cost.
2. Reduced environmental impact.
3. The need for specific properties unattainable with portland cement concrete.
This desire has resulted in a significant number of new materials coming into the market. Alternative cements (inorganic cements that can be used as complete replacements for portland or blended hydraulic cements) include geopolymers, activated glassy cements, activated fly ash cements, activated slag cements, calcium aluminate cements, calcium sulfoaluminate cements, magnesia cements, CO2-cured cement, and more.
Most alternative cements are not currently covered by standard material specifications such as those for portland cement or blended cements (for example, ASTM C150, Standard Specification for Portland Cement). Therefore, ACI 318-19 has added provisions allowing the use of alternative cements only under strictly controlled conditions. Once a design professional has identified the concrete properties necessary for a given application, the material suppliers must provide detailed information on their own products’ expected influence on the concrete, using their experience with portland cement concrete as a benchmark. The basic material properties that must be established for any alternative material to be used in structural concrete include:• Chemical composition.
• Loss on ignition.
• Air content of mortar.
• Fineness (or other measure of particle size).
• Autoclave expansion.
• Compressive strength.
• Heat of hydration.
• Sulfate resistance.
• Other properties dependent on the actual material.
The material specifications in this and previous versions of ACI 318 apply only to hydraulic cement, which is defined as a cement that sets and hardens by chemical reaction with water and is capable of doing so under water. Many alternative cements, however, do not rely on a chemical reaction with water. Standards for cements apply to the cementitious material considered alone or in a mortar, and not to mixtures that might be considered structural concrete Therefore, even if an alternative cement meets the chemical or physical requirements of one of the applicable provisions in ACI 318-19 (which are based on ASTM standards for hydraulic cement), it may not automatically be appropriate for use in creating structural concrete.
For nonhydraulic materials, the relationships between water-cementitious materials ratio (w/cm) and strength and durability may not be the same as for portland cement concrete. This point has implications for design and concrete production as well as for specifying concrete durability.
Suppliers can expect to be required to conduct laboratory and field testing assessing a material’s effect on a mixture including:• Thermal cracking.
• Volume stability.
• Elastic properties.
• Corrosion of metals.
• Reactions with aggregates.
• Resistance to freezing and thawing, chemicals, or high temperatures.
Concrete producers must also provide evidence that concrete made with alternative cements will behave consistently during batching, transportation, and placing. Concrete mixtures made with the alternative cement will also be required to undergo testing to determine how production should be modified (if at all). For example, considerations should include:• Safety.
• Storage of materials.
• Mixture proportioning.
• Compatibility with admixtures and ability to entrain air.
• Mixing time and restrictions on time in the mixer drum.
• Testing of fresh and hardened concrete properties.
Structural design and performance should also be adequately tested, including:• Axial, compressive, flexural, shear, and torsional strength.
• Ultimate strain and stress-strain relationship.
• Volume change properties (drying, thermal, creep, and shrinkage).
• Modulus of elasticity.
• Bond of reinforcement
• Strain compatibility of concrete and reinforcement.
In addition to replacing portland cement with alternative cements, another strategy for making concrete more sustainable has been to incorporate recycled materials into the mixture. As with its treatment of alternative cements, ACI 318-19 allows for crushed hydraulic-cement concrete or recycled aggregate to be used in a structural concrete mix if it is approved by the licensed design professional and the building official. Here, too, the burden is on the supplier to show that concrete using alternative aggregates meets all project requirements for structural and durability properties, with performance criteria and categories being similar to those listed above for alternative cements. Furthermore, the team must show that the aggregate supply will be consistent throughout the life of the project.
Once data for a given product or process has been generated, it should be applicable for many other circumstances. In any case, when an alternative is proposed as a substitute for a specified cementitious material or aggregate during the bidding or construction phase of a project, the supplier’s ability to have data readily available to confirm fundamental structural characteristics becomes critical to avoid delays. More information on test methods can be found in ITG-10.1R-18, Report on Alternative Cements. The ITG-10, Practitioner’s Guide provides guidance for those looking to implement alternative cements and discusses currently available and emerging alternative cements.
Other New Materials
With the release of ACI 318-19, the use of shotcrete is explicitly addressed in the ACI 318 code for the first time. The new content was developed by updating relevant provisions from the International Building Code (IBC), with input from the American Shotcrete Association and ACI Committee 506 on Shotcrete. In the future, ACI standards will govern the use of shotcrete and the IBC will drop all references to it.
High-strength rebar is another material advancement addressed in ACI 318-19. Current U.S. building codes limit rebar strength based on decades-old research, with most reinforcement used in concrete construction in the United States being Grade 60. Progress in metallurgy, however, has resulted in production of rebar that is almost twice as strong as it was several decades ago. This stronger rebar is able to transfer much greater stress but may also lack some of the properties of weaker steels, such as minimum strain-hardening and elongation. Recognizing this, ACI 318-19 includes new requirements for material properties of higher-strength steels. The many updates in the new code that address high-strength rebar are expected to support the use of these bars in structural concrete construction, which will reduce congestion in heavily reinforced members, improve concrete placement, and save time and labor.
Post-installed concrete screw anchors are increasingly being used in place of expansion anchors or cast-in anchors, and this anchor type is now recognized in ACI 318-19. The document also introduces provisions for shear lugs, which are a steel element that is welded to a base plate. Shear lugs are usually used at the base of columns to transfer large shear forces to a foundation.
New Designs and Detailing for Seismic Resistance
ACI 318-19 raises the limits on the specified strength of reinforcement in shear wall and special moment frame systems. The new standard allows Grade 80 reinforcement for some special seismic systems and no longer allows Grade 40 rebar to be used in seismic applications. Shear walls can employ rebar in Grades 60, 80, or 100. Special moment frames can use Grades 60 or 80. Hoops and stirrups in special seismic systems used to support vertical reinforcing steel have a tighter specified spacing to prevent the vertical bars from buckling.
The 1994 Northridge earthquake in California, along with more recent earthquakes in Chile and New Zealand, led to research studies focused on improving seismic design. ACI 550 standards that were developed to improve the response of precast concrete diaphragms and connections are now referenced by ACI 318-19. These include new requirements for the design and detailing of precast concrete diaphragms, particularly the connections between precast elements.
Other changes in the new ACI 318-19 that address seismic design are requirements for boundary elements of special structural walls. (Boundary elements, which typically occur around wall edges, corners or openings, provide longitudinal or transverse reinforcement to confine concrete and provide longitudinal bar support.) Whereas previous standards permitted the use of crossties with 90-degree hooks at one end, ACI 318-19 specifies that all crossties for special boundary elements now must have 135-degree hooks at both ends. New provisions also limit the aspect ratio of hoops in the boundary element, restrict the locations of lap splices near intended plastic hinge zones, and require some walls to satisfy minimum longitudinal reinforcement requirements to avoid brittle fracture of under-reinforced walls.
Clarifying Certification Requirements
ACI 318-19 now identifies qualification training programs for inspectors and installers and lists the certification requirements. All inspectors are now required to be certified if there is an appropriate certification program available.